The present invention relates to wet-rotor circulators and, more specifically, to improved rotor assemblies and methods of fabrication thereof for use in wet-rotor circulators.
Wet-rotor circulators are motor/pump combinations in which the motor rotor is connected directly to the pump impeller and rotates in the pumped fluid. Typically, the circulator includes a pump housing with an impeller rotatably mounted in a pumping chamber with the impeller connected directly to the rotor shaft. The rotor assembly is typically supported in sleeve bearings and is mounted within a fluid impermeable jacket that is sealed against the pump housing so that the rotor is surrounded by and rotates in the pumped fluid during its operation but is otherwise fluid isolated from the stator assembly by the fluid impermeable jacket. The jacket is typically fabricated from a low-permeability material and is mounted within the motor stator so that the rotating magnetic field provided by the stator assembly passes through the jacket wall to cause rotation of the rotor and the connected pump impeller. Circulators of this type are typically used in circulating hot water in home heating systems as well as other applications.
Since the rotor and connected impeller are the only moving parts in the wet-rotor circulator, the operating life of the circulator is a function of the operating life of the bearings that support the rotor shaft. One of the factors that affects bearing life in the wet rotor environment is the axial alignment between the axes of the sleeve bearings that support the rotor shaft and the axis of the rotor shaft. Preferably, the axes of the support bearings should be parallel (that is, coaxial) with the axis of the rotor shaft, since a skewed axial relationship can diminish bearing life. In general, it is possible to economically manufacture the rotor, the support bearings, and the jacket to tolerances which nominally provide the desired axial alignment between the rotor and the bearings. However, problems have been encountered in the past in attempting to affix or otherwise secure the support bearings in place within the jacket to provide a finished circulator having the desired bearing-to-shaft axial alignments. Since the jacket is typically fabricated as a cup-like, deepdrawn, thin-shell component and, accordingly, has limited structural rigidity, bearing retaining techniques which involve the application of a force against the jacket are generally unsuitable. For example, crimping the exterior shell of the jacket to the bearings is generally not successful since the very nature of the crimping operation, i.e., the forcible application of a crimping tool to the jacket, causes distortion of the jacket so that it is difficult to establish and maintain the desired parallel alignment between the axis of the rotor shaft and the axes of its support bearings. Accordingly, crimped-jacket rotor assemblies can be prone to premature bearing failures.
In the past, the problem of affixing the bearings in place within the jacket has been met by applying a curable cement to appropriate locations in the interior of the jacket, inserting the rotor and assembled bearings into the jacket, establishing the desired axial relationships with the aid of an assembly jig, and then curing the cement to achieve the permanent assembly. Since this method does not involve the application of force to the as-manufactured piece-parts, the cemented bearing method does provide a finished circulator having the desired axial relationships. However, problems have been encountered with this type of assembly. As a practical matter, substantial variations in cement performance can occur because of run-to-run manufacturing variations and because of temperature and humidity changes during the preparation of the cement and its application to the bearings. Also, the cement must be cured over a period of time, which curing period is relatively greater than conventional mechanical fabrication techniques.
Accordingly, there is a need for an inexpensive assembly method for establishing the desired parallel axis relationship between the rotor shaft and its support bearings and for quickly securing the rotor bearings in place within the jacket in such a manner that the so-established relationship between the rotor shaft and its bearings will not be adversely affected.